Arsenic is a historical poison dramatized in literature, utilized in commerce and occurring in nature with other important resources. Arsenic compounds have long been used to prepare insecticides for agricultural and commercial uses and as wood preservatives. Arsenic naturally occurs in the waste of mining and smelting operations for minerals. These arsenic compounds find their way, or are dumped, into water impoundments and ground water. The problems of the presence of arsenic in ground water, surface water, and process waste streams are wide spread and troublesome.
The problems of groundwater, surface water, leachates, and process waste streams containing unacceptable levels of arsenates and other contaminants abound. Typical contaminated sites include manufacturers of arsenicals, certain wood treatment plants and other users of arsenicals, mining operations and leachates from abandoned mine tailings containing arsenates, various metallurgical operations, fly ash leachate from plants burning arsenic contaminated coal, and areas of heavy arsenicals application in agricultural, such as for cotton defoliation.
Such water is rarely, if ever, contaminated only by simple arsenates, since the sources of contamination typically contain additional toxic materials such as other forms of arsenic, e.g., arsenic (III) compounds organoarsenicals manufactured or naturally produced by chemical and/or biochemical process in soil or water; toxic metal ions, e.g., Cu.sup.2+ and Cr.sup.3+ from "CCA Process" wood treatment; or "heavy metal ions" from mine tailings leachates, etc., toxic anions, e.g., cyanide from certain metallurgical processes; and/or toxic organic species, e.g., biocides produced as additional products by arsenicals manufacturers, or "creosote" components from certain wood treatment operations.
Accordingly, any effective treatment of such contaminated water must address an entire spectrum of toxic species that may include anions, cations, and electrically neutral species. Effective treatment must reduce the concentrations of all of the toxic or hazardous contaminants to the very low levels required for reuse of treated water and/or to meet the increasingly stringent requirements of regulatory agencies for permitted sewer or surface discharge. Solving the arsenic contamination problem alone is not enough. It is not enough, either, to solve other contamination problems without addressing the arsenic problem.
Conventional methods of wastewater treatment are rarely, if ever, suitable for water contaminated by arsenates and other toxic or hazardous species, except as a "primary treatment" part of the new process described herein, especially when treated water must meet or exceed "drinking water" quality standards (1991 level, 50 ppb arsenic).
Many problems accompany conventional attempts to solve this problem, most of which depend upon chemical precipitations. For example, precipitation processes are taught in U.S. Pat. Nos. 4,244,927, 4,244,734, and 4,149,880, particularly for removing metals from the presence of arsenic-contaminated materials. Also U.S. Pat. No. 4,201,667 describes a precipitation process for removing arsenic from aqueous mediums in presence of phosphate by a coprecipitation. Some phosphate itself may remain in such amounts that disposition of the remaining aqueous materials may be either unusable or prohibited from discharge to the environment without additional treatment. Other patents describe processes for recovering arsenic in the presence of various desirable metals such as zinc and copper and the like. See, for example, U.S. Pat. Nos. 4,240,826, 4,220,627, and 3,171,735, as well as 3,436,177. Neither chemical precipitations nor absorption methods are well suited for "mixed species" wastewater treatment, and it is not uncommon to find treatment schemes that compromise on precipitation or absorption parameters to effect the "best over-all removal efficiency" in a way that one, or more, species are left well above proper discharge limits.
Arsenates pose an almost unique problem in water treatment because of the pH dependence of the relative concentrations of arsenate species. In even the simplest case, there are four interactive species in arsenate solutions: arsenic acid (H.sub.3 AsO.sub.4, dominant at pH &lt;3), dihydrogen arsenate ion (H.sub.2 AsO.sub.4-, dominant around pH 6), hydrogen arsenate ion (HAsO.sub.4.sup.2-, dominant around pH 10), and arsenate ion (AsO.sub.4.sup.3-, dominant at pH 12 and above). In intermediate pH ranges, significant concentrations of two differently protonated species occur. Further, precipitation and oxidation/reduction reactions of arsenates are highly dependant on pH. Consequently, treatments to remove arsenates from water face many more problems than those for removal of other wastes, and removal of arsenates to very low levels in the presence of other contaminants which must be removed is exceedingly difficult, and, insofar as is known, has not been previously accomplished.
Attempts have been made to reduce the arsenic content in an aqueous medium through precipitation processes such as described in U.S Pat. No. 4,201,667 for example, and while such removal is successful to a certain extent, large quantities of phosphate ion must be present to satisfactorily co-precipitate with the arsenic. Much of the development of the solutions to the problems for the removal of arsenic relate to attempts to recover other minerals, particularly copper as described in U.S. Pat. Nos. 3,436,177, 4,149,880, and 4,220,627, not to clean the water. Other processes for recovering other metal values in the presence of arsenic are described in, for example, U.S. Pat. Nos. 4,244,734 and 4,240,826. Arsenic metal values have been precipitated by treatment of the solutions with iron salts, lime, magnesium oxides, heavy metals such as barium and titanium, for example, and aluminum to mention only a few of the attempts. Almost uniformly attempts of this nature have failed to recover water that could be considered potable from the presence of the arsenic contamination. The present limit with respect to potable water is 50 parts per billion (50 ppb).
Ion-exchange treatment alone of a complex "raw" wastewater, or even of effluent from a precipitative pretreatment, will typically face low efficiency and high cost and may fail to achieve acceptable levels of particular species, simply because of the high ionic strength and the composition from the multiplicity of ions commonly found in such solutions, and will fail to remove neutral species.
Accordingly, it is an object of this invention to provide a process whereby arsenic may be removed from aqueous waste streams to the extent that the water may be returned to useful purposes.
It is a further object of this invention to provide a process whereby the arsenic becomes more concentrated and therefore more economically disposed of through environmentally safe procedures.
Most of the processes in the previously cited patents were designed to remove arsenic in metallurgical operations, routinely to levels at about 1 ppm. Large excesses of precipitating agent are often required which produces voluminous amounts of sludge, often gelatinous and difficult to dewater. Thus, attempts to achieve low levels of arsenic by these processes are generally unsuccessful, and excess precipitating agents pose major problems for any subsequent reverse osmosis treatment to achieve satisfactory arsenic levels.
It is a further object of this invention to provide an environmentally safe, economically sound process for removal of arsenic from aqueous lakes, streams, and aquifers.
It is a still further object of this invention to provide an economic and environmentally safe process for treating aqueous plant effluent streams to provide on-site protection of the downstream environment.